[1] Field of the Invention
The present invention relates to a phosphor as well as a plasma display panel using the phosphor, in particular to a zinc silicate phosphor activated by manganese.
[2] Related Art
Manganese-activated zinc silicate phosphors (hereinafter, referred to as “ZSM phosphor”) used for plasma display panels (hereinafter, “PDP”) and fluorescent lamps are commonly used as green phosphors having a high color purity and a high luminous efficiency.
ZSM phosphors have a base material represented by a chemical formula of Zn2SiO4, however it is considered desirable to arrange the composition of the base material so that the ratio of the number of atoms between zinc (Zn) and silicon (Si) becomes 1.5, where more silicon atoms are included than in the stoichiometric composition (e.g. refer to “Keikohtai Handbook (Handbook of Phosphors)” edited by the Phosphor Research Society and published by Ohmsha on 25 Dec. 1987, pp. 219-220). This literature argues that, while the ratio of the number of atoms between Zn and Si in the stoichiometric composition of Zn2SiO4 is 2, it is essential that the base material of ZSM phosphors used for PDPs and fluorescent lamps includes more silicon atoms than in the stoichiometric composition in order to achieve a high luminous efficiency.
In order to manufacture ZSM phosphors, a method including the following processes is used: combining a source of silicon represented by silicon dioxide, a source of zinc represented by zinc oxide, and a source of manganese represented by manganese carbonate in a compounding ratio where the silicon component is included at a slightly higher rate than in the stoichiometric composition ratio, for the above reason; and heat-treating (baking) the combined result at a temperature of approximately 1200° C. in the atmosphere or in a reduction atmosphere (e.g. refer to “Keikohtai Handbook (Handbook of Phosphors)” edited by the Phosphor Research Society and published by Ohmsha on 25 Dec. 1987, pp. 219-220). In addition, since the heat treatment is conducted in a high temperature atmosphere as described above, the zinc component tends to sublime from the surface of each phosphor particle during the manufacturing operation. As a result, more silicon is present particularly in the composition of the surface of each phosphor particle, as compared to the average composition of the entire particle. It is considered that part of silicon at the surface of the particles of ZSM phosphors is present as silicon dioxide (e.g. refer to “Keikohtai Handbook (Handbook of Phosphors)” edited by the Phosphor Research Society and published by Ohmsha on 25 Dec. 1987, pp. 219-220).
Regarding ZSM phosphors, there is an issue of time-lapse degradation, in which the luminous efficiency decreases as the lighting time proceeds, depending on the status of use. In order to address the issue, a method has been proposed, for example, to form a layer of a silicon nitride compound on the surface of the phosphor particles when ZSM phosphors are used for fluorescent lamps (e.g. refer to Examined Patent Publication No. H06-62944). Another proposed method is shown, for example, in Japanese Patent No. 2811485 and U.S. Pat. No. 4,728,459, in which tungsten oxide is added during the manufacture of phosphors to thereby reduce the time-lapse degradation of the phosphors.
However, in various uses such as for PDPs and fluorescent lamps, it is difficult for conventional technologies, including the suggestions discussed in the above-mentioned references, to achieve compatibility between obtaining a high luminous efficiency at the beginning of the drive and reducing the degradation after a long driving period. For example, the technology suggested in the above Examined Patent Publication No. H06-62944 is practically difficult to apply to PDPs and the like because it is examined, focusing only on fluorescent lamps. That is, in a PDP, an electric discharge occurs in a very small space when the PDP is driven, and short-wavelength and high energy ultraviolet radiation exerts an impact on the phosphor. Accordingly, it is, in fact, difficult to provide protection for the phosphor even if the technology suggested in the above reference is adopted. In addition, adopting this technology for a PDP will end up creating another problem of significantly deteriorating luminous efficiency of the PDP since ultraviolet radiation for exciting the phosphor as well as visible radiation generated by emission are absorbed by the compound layer coating the phosphor particles.
It is expected that the technologies suggested in the above Japanese Patent No. 2811485 (pp. 1-2) and U.S. Pat. No. 4,728,459, involving a process of adding tungsten oxide during the manufacture, bring about an effect of reducing the time-lapse degradation in a degree. However, it is considered that they are not yet sufficient to achieve both a high luminous efficiency and a reduction in the time-lapse degradation. That is, if tungsten oxide is added in the amount suggested in these references, the luminous efficiency declines. On the other hand, if the addition amount of tungsten oxide is reduced, the effect of reducing the time-lapse degradation is not adequately obtained although the phosphor maintains the luminous efficiency at the level equivalent to that of a phosphor with no tungsten oxide added. Thus, the method employing tungsten oxide addition has a trade-off between the luminous efficiency and the reduction in the time-lapse degradation.